Digester system



Jain; 16, 1934. A. .CHRLSTENSEN' DIGESTER SYSTEM W ab Jan. 16, 1934. A g s E 1,943,289

DIGESTE R SYSTEM .FilBd D80. 10. 1932 2 ShOBiS-Sheel. 2

Patented Jan. 16, 1934 UNITED STATES DIGESTER SYSTEM Andreas Christensen, Rhinelander, Wis., assignor to Fibre Making Processes, Inc., Chicago, 111., a

corporation of Illinois Application December 10, 1932 Serial No. 646,600

12 Claims.

This invention has to do with a system for digesting wood and like materials in the production of pulp.

One of the main objects of my invention is to provide a digester system comprising a plurality of digesters having associated therewith means including indirect heaters for heating the liquor during the cooking operation, means being provided whereby liquor may be transferred from certain of the digesters to certain other of the digesters as desired or as conditions may require. A further object is to provide simple and eificient means for transferring liquor from certain of the digesters to other of the digesters, or for supplying additional liquor to the digesters, selectively, such means being supplementary to the local liquor circulating systems of the respective digesters. It is also an object of my invention to provide means for preventing entry of minerals into the tubes of the indirect heaters. Further objects and advantages of my invention will appear from the detail description.

In the drawings:

Figure 1 is a plan view of a digester system embodying my invention;

Figure 2 is an elevation, partly broken away and in section, of one of the digesters and the local liquor circulating and heating system associated therewith, and associated parts;

30 Figure 3 is a fragmentary sectional view, on an enlarged scale, through the upper portion of one of the indirect heaters, taken substantially on line 33 of Figure 2, the pump being omitted.

The system of my invention includes three 35 digesters 4 connected together in a set or series to accomplish the desired results, as will be more fully explained. While I have shown, by way of example, three digesters as constituting a set, the number of digesters included in a set or 4 series may be varied within limits, as desired.

The digesters are, in general, of known type and each includes an outer casing or shell 5 and a lining 6 which is resistant to the acid used in the digester. A charging neck 7 is provided at the top of the digester and is closed by a removable cover 8. A blow-out fitting 9 is secured to the bottom of the digester for convenience in blowing the pulp therefrom and in washing the digester. Pipes 10 and 11 are connected to the 5 fitting 9, pipe 11 being connected to one end of the fitting and pipe 10 being connected to the fitting between the ends thereof. Pipe 10 is provided with a-suitable valve 12 and pipe 11 is provided with a check valve 13 of known type which permits of flow of liquor through pipe 11 to fitting 9, but prevents reverse flow of the liquor in pipe 11. A discharge pipe 14 is connected to the other end of fitting 9, this pipe being provided with a suitable valve 15. The construction of the digester is, in general, known in the art and need not be illustrated nor described here in greater detail.

A tubular screen 16 is supported by brackets, one of which is shown at 17, in the upper portion of the digester. Each of the brackets is suitably secured to the wall of the digester, as by means of a bolt 18. The screen extends around the interior of the digester for the greater portion of the circumference thereof. Preferably, the screen employed and the mounting therefor correspond to the screen and mounting disclosed in my copending application for sulphite pulp digester system, Serial No. 617,572, filed June 16, 1932, though any other suitable screen and mounting therefor may be employed. 75

Screen 16 is suitably connected, as by means of a sleeve 19, to a take-off pipe 20, the upper end of which extends through the wall of the digester. This pipe 20 is provided with a control valve 21 of known type and which may be operated in any suitable or known manner. The lower end of pipe 20 is connected to the intake of a pump 22 of known type, driven by an electric motor 23. The discharge of pump 22 is connected to a header 24 at the upper end of a heater 25. This heater is of known type and comprises a lower header 26 to which pipe 11 is connected, a casing 27 extending between the headers and enclosing vertical tubes 28 which establish communicationbetween the upper header and the lower header. Casing is provided at its upper end with a steam inlet nipple 29 to which is con nected a pipe 30 leading to a suitable source of supply of steam or other heating medium.. Adjacent its lower end the casing 25 is provided with a discharge nipple 31 to which is connected a pipe 32 for discharging condensate from the casing. The steam admitted to the casing flows around and between the tubes 28 in intimate contact therewith and serves to heat the liquor flowing through these tubes to the desired temperature. The tubes 28 are of proper interior diameter and sufiicient in number to avoid objectionable interference with fiow of the liquor circulated through the system, the capacity of the pump being such as to force the liquor through the tubes at appreciable velocity.

The digester is filled with wood chips and with cooking liquor or acid. in the usual manner. The acid, which is a bisulphite solution, is usually a bisulphite of calcium or dolomitic lime, and free sulphur dioxide in excess, Ca or Mg (HSOa) 2+SOz, the sulphur dioxide (S02) combining with the water of the solution to form sulphurous acid (H2803), and the sulphurous acid in turn dissolving the calcium or magnesium to form calcium or magnesium bisulphite. During the cooking operation, the acid liquor is withdrawn from the upper portion of the digester and is passed through the heater and thence, by means of pipe 11 and fitting 9, to the lower portion of the digester in which the heated liquor flows upwardly to complete the path of circulation, thus heating the contents of the digester. The pressure within the digester and the temperature of the contents thereof may be varied to meet the requirements of the different types of pulp to be made, as is known in the art. The variations in pressure and temperature during the cooking operation, and the reasons for such variation, are known by those skilled in the art and need not be set forth here in detail.

It is known that, when the acid liquor is heated, the free sulphur dioxide, owing to the unstability of the solution, tends to separate from the liquor and to leave a solution of calcium or magnesium bisulphite. This is objectionable, since insoluble lime salts would tend to collect upon the walls of the heater tubes and, in time, form a thick layer or coating which would materially reduce the efficiency of the heat exchanger and, eventually, would completely stop the tubes. This tendency of the S02 to separate from the liquor upon heating of the latter may be greatly reduced or completely avoided by using strainers and an intake pipe to the circulating pump, of sufficient capacity to guard against a drop in pressure, and maintaining the liquor under considerable pressure when it is heated. The pump 22 delivers to the top header 24 of the heater 25 so that the liquor is forced downwardly through the tubes 28 under considerable pressure, the direction of flow of the liquor through the tubes being contra to the direction in which the liquor in these tubes tends to flow due to the temperature differences of the liquor. The tendency to upward flow of the liquor is thus utilized to subject the liquor in the tubes to a slight pressure additional to the pressure exerted by the pump. with the result that the liquor in the tubes 28 is maintained under sufficiently high pressure, while being heated and under normal conditions, to prevent separation of the sulphur dioxide from the liquor. This pressure, combined with the flow of the liquor through the tubes 28 under appreciable velocity, is ejective in preventing deposition of calcium or other minerals within the heater tubes 28, with the result that the efliciency of the heater remains unimpaired and the flow of the liquor therethrough is not interfered with under normal conditions.

In the operation of the digester. lime salts are deposited on the inside of the strainer. These salts work loose and flow with the liquor into upper head 24 of the heater. An inclined screen 33 is suitably supported within head 24, as by means of properly disposed lugs 34 carried by the head at the inner face thereof. This screen is disposed between the liquor inet of head 24 and the upper ends of tubes 28. The liquor entering the head 24 passes through the screen before entering the tubes. The screen is of proper mesh, or is provided with openings of proper size, to prevent passage through the screen of pieces of lime scale that might form on the inner walls of thestrainer and from time to time break off and be carried into the heater by the flow of the liquid. This scale will only form when the digesting of the wood is done by means of an acid with a high lime content and low total acid content. The screen is only provided to take care of unusual operating conditions and also an emergency as the lime scale mentioned will not form when the acid is of proper composition and operations have been adjusted to conform to this method of cooking. This assures that the liquor flowing into the tubes 28 is free of particles of lime large enough to lodge in the tubes, thus avoiding possibility of clogging of the tubes or objectionable interference with flow of liquor therethrough. Preferably, the screen 33 is inclined downwardly toward the inlet of head 24, which increases the effectiveness of the screen and assures that the bulk of the particles of lime will flow downwardly by gravity toward the lower end of the screen, leaving the greater portion of the screen unobstructed. By removing cover 24a of head 24, ready access is had to screen 33 and to the interior of the head for removing the limedeposit and for cleaning or replacing the screen.

When cooking wood. which is unusually dry, more of the acid liquor is absorbed by the chips than is ordinarily the case, with the result that the level of the liquor may be lowered sufficiently to leave the chips at the top of the digester uncovered. These chips, if permitted to become dry, are apt to char.and turn black, which is highly objectionable. To avoid this, I preferably provide a return pipe 35 having control valves 36 and'3'7 of known type. Pipe 35 is connected, at its lower end, to head 24 of the heater and, at its upper end. to a spray head 38 disposed within neck '7 of the digester. A branch pipe 39 connects pipe 35 of each of the digester circulating systems to a pipe line 40 which extends along the series of digesters. A control valve 41, of known type, is disposed in each of the branch pipes 39. With valves 36 and 3'7 open, and valve 41 closed, acid liquor is delivered from pump 22 to spray head 38 which latter serves to direct the liquor in a spray and downwardly onto any exposed chips so as to maintain the latter in a moist condition and eliminate possibility of charring thereof. The pipe 35 and the associated spray means may be used at all times, or only when cooking unusually dry wood, as desired. By closing valve 36. flow of liquor through pipe 35 is prevented and the liquor is circulated through the local circulating system comprising the strainer 16. pipe 20. the heater, pipe 11. and fitting 9 back to the digester. The pump 22 is of adequate capacity to supply the liquor to pipe 35. when used, in ample quantity to keep any exposed chips moist while also forci'ng liquor through the heater tubes at sufiicient velocity and under proper pressure to eliminate any possibility of coating of the tubes with lime salts.

The arrangement of the local liquor circulating system. including the heater and the return pipe and associated parts. is disclosed in my copending application above identified. I have also disclosed in my co-pending application automatic control meansfor preventing excessive pressure within the digester. I contemplate the use of an automatic pressure control, of suitable type, in conjunction with the digester system of the instant application, though such automatic regulating means is not essential thereto.

The pipe line 40 connects the return pipes of all of the digesters of the series, by means of the branch pipes 39. One end of pipe line is connected to the branch pipe 39 of the last digester of the series, and the other end of the pipe line is connected to the outlet of a booster pump 42 of known type driven in a suitable manner, as by means of an electric motor 43. A control valve 45, of known type, is disposed in pipe line 40 between pump 42 and the first digester 4. The inlet of pump 42 is connected by a pipe 44 to a suitable source of supply of acid or acid liquor. By closing valve 36 and opening valves 39 and 45, valve 3'7 remaining open, acid liquor can be supplied to any of the digesters 4, selectively, from the exterior source to which pipe 44 is connected. This additional liquor is sprayed into the upper end of the digester through the spray head 38 in the same manner as the liquor which is supplied to this spray head through pipe 35 from pump 22, it being noted, however, that the additional liquor thus supplied to the digester enters the latter exterior of the local circulating system for heating the liquor.

In practicing my invention, the digesters of the series or set are so timed that the cooking process is at different stages in the different digesters. The temperatures and the pressures in the digesters are therefore different, and it is possible to transfer liquor from a digester within which the pressure is relatively high to another digester of the series within which the temperature isrelatively low. For example, if it be assumed that the pressure within the central digester shown in Figure l is appreciably higher than the pressure within either of the end digesters, liquor can be transferred from this central digester to either one or both of the end digesters. In

' order to accomplish this, valve 36 of the selected end digester return pipe is closed and valve 41 of the corresponding branch pipe 39 is opened, valve 37 remaining open and valve 45 of the pipe line 40 being closed. Valve 3'? of the return pipe of the central digester is then closed and valve 41 of the corresponding branch pipe 39 is opened, valve 36 remaining open. Since the pressure within the central digester is appreciably higher than the pressure within the selected end digester, liquor will flow through pipe 20 of the central digester to head 24 of the corresponding heater 25 and hence through pipe 35 and branch pipe 39 to pipe line 40, this liquor then flowing through pipe line 40 to branch pipe 39 of the selected end digester and thence through pipe 35 to spray head 38, which delivers the liquor to the end digester. During this transfer of liquor from the digester of higher pressure to the digester of lower pressure, pump 22 of the circulating system of the digester of higher pressure may remain in operation, this pump creating pressure which effects or facilitates the desired transfer of the liquor. If the pressure Within the central digester or digester of higher pressure is sumciently high to effect the desired transfer of liquor without the assistance of pump 22, this pump may remain in operation or not, as desired. It is to be noted that the liquor transferred to the low pressure digester enters the latter exterior of the local circulating system therefor so that circulation of the liquor through this system is not interfered with and, if desired, the circulation of the liquor through the local system of the higher pressure digester may be continued. If the two end digesters of the system are at about the same internal pressure, the central digester being at a higher internal pressure, liquor may be transferred from the central digester to both of the end digesters simultaneously, if desired. It will be understood, of course, that the stages of the cooking process in the digesters may be so related that the central digester may be at lower pressure than the end digesters, or the pressure may decrease from one end of the series of digesters toward the other end thereof.

What I claim is:-

1. In combination in a digester system, a plurality of digesters for containing digesting liquor and material to be digested, independent liquor circulating systems associated with the respective digesters and each including an indirect heater and pump means for circulating liquor from the digester to and through the heater and back to the digester, and means supplemental to the circulating systems for selectively delivering liquor from one of the digesters to the other digesters.

2. In combination in a digester system, a plurality of digesters for containing digesting liquor and material to be digested, independent liquor circulating systems associated with the respective digesters and each including an indirect heater and pump means for circulating liquor from the digester to and through the heater and back to the digester, and means supplemental to the circulating systems for selectively delivering liquor from one of the digesters to the other digesters, and for selectively delivering liquor to the digesters from an exterior source, optionally.

3. In combination in a digester system, a plurality of digesters for containing digesting liquor and material to be digested, independent liquor circulating systems associated with the respective digesters and each including an indirect heater and pump means for circulating liquor from the digester to and through the heater and back to the digester, return pipes connecting the respective systems to the associated digesters for delivering to the latter and exterior of the circulating system liquor from said system, and means supplemental to the return pipes for selectively closing the latter to the circulating systems and selectively delivering to some of the digesters, through said pipes, liquor from the other of said digesters.

4. In combination in a digester system, aplurality of digesters for containing digesting liquor and material to be digested, independent liquor circulating systems associated with the respective digesters and each .including an indirect IOU heater and pump means for circulating liquor from the digester to and through the heater and back to the digester, return pipes connecting the respective systems to the associated digesters for delivering to the latter and exterior of the circulating system liquor from said system, and means supplemental to the return pipes for selectively closing the latter to the circulating systems and selectively delivering to some of the digesters, through said pipes, liquor from the other of said digesters, and for selectively delivering liquor to the digesters from an exterior source, optionally.

5. In combination in a digester system, a plurality of digesters for containing digesting liquor and material to be digested, independent liquor circulating systems associated with the respective digesters and each including an indirect heater and pump means for circulating liquor from the digester to and through the heater andv pipe line connected to the return pipes for establishing communication therebetween, and valve means for cutting off communication between the respective return pipes and circulating systems and establishing communication between the respective return pipes and the pipe line and cutting off communication between the pipe line and the respective return pipes and establishing communication between said pipes and said systems, optionally.

6. In combination in a digester system, a plurality of digesters for containing digesting liquor and material to be digested, independent liquor circulating systems associated with the respective digesters and each including an indirect heater and pump means for circulating liquor from the digester to and through the heater and back to the digester, return pipes connecting the respective systems to the associated digesters for delivering to the latter and exterior of the circulating system liquor from said system, a pipe line connected to the return pipes for establishing communication therebetween, valve means for cutting off communication between the respective return pipes and circulating systems and establishing communication between the respective return pipes and the pipe line and cutting off communication between the pipe line and the respective return pipes and establishing communication between said pipes and said systems, optionally, and means for supplying to the pipe line liquor under pressure and from a source exterior to the digesters.

7. In combination in a digester system, a plurality of digesters for containing digesting liquor and material to be digested, independent liquor circulating systems associated with the respective digesters and each including an indirect heater and pump means for circulating liquor from the digester to and through the heater and back to the digester, return pipes connecting the respective systems to the associated digesters at the top thereof for delivering to the latter and exterior of the circulating system liquor from said system, valves in the respective return pipes for controlling communication between the same and the respective circulating systems, a pipe line connected to the return pipes, and valves controlling communication between the respective return pipes and said pipe line.

8. In combination in a digester system, a plurality of digesters for containing digesting liquor and material to be digested, independent liquor circulating systems associated with the respective digesters and each including an indirect heater and pump means for circulating liquor from the digester to and through the heater and back to the digester, return pipes connecting the respective systems to the associated digesters at the top thereof for delivering to the latter and exterior of the circulating system liquor from said system, valves in the respective return pipes for controlling communication between the same and the respective circulating systems, a pipe line connected to the return pipes, valves controlling communication between the respective return pipes and said pipe line, and valve controlled means for delivering to the pipe line liquor under pressure and from a source exterior to the digesters.

, 9. In combination in a digester system, a plurality of digesters for containing digesting liquor and material to be digested, independent liquor circulating systems associated with the respective digesters and each including an indirect heater and pump means for circulating liquor from the digester to and through the heater and back to the digester, return pipes connecting the respective systems to the associated digesters for delivering to the latter and exterior of the circulating system, liquor from said system, a pipe line, branch pipes connecting said pipe line to the respective return pipes, valves in the branch pipes for controlling communication between the latter and the pipe line, and valves in the return pipes between said branch pipes and the circulating systems for controlling communication between the latter and said return pipes.

10. In combination in a digester system, a plurality of digesters for containing digesting liquor and material to be digested, independent liquor circulating systems associated with the respective digesters and each including an indirect heater and pump means for circulating liquor from the digester to and through the heater and back to the digester, return pipes connecting the respective systems to the associated digesters for delivering to the latter and exterior of the circulating system liquor from said system, a pipe line, branch pipes connecting said pipe line to the respective return pipes, valves in the branch pipes for controlling communication between the latter and the pipe line, valves in the return pipes between said branch pipes and the circulating systems for controlling communication between the latter and said return pipes, and valve controlled means for delivering to the pipe line liquor under pressure and from a source exterior to the digesters.

11. In combination, a digester for containing digesting liquor and material to be digested, a liquor circulating system including an indirect heater and pump means for circulating liquor from the digester to and through the heater and back to the digester, a return pipe connected to .the circulating system and to the digester for delivering to said digester and exterior of the circulating system liquor from said system, a liquor supply pipe connected to the return pipe, valve means for establishing and cuttingoif communication between the supply pipe and the return-pipe, and valve means in the return pipe between the connection thereof to the supply pipeand the circulating system for establishing and cutting oiT communication between the latter and said return pipe.

12. In combination, a digester for containing digesting liquor and material to be digested, a liquor circulating system including an indirect heater and pump means for circulating liquor from the digester to and through the heater and back to the digester, a return pipe connected to the circulating system and to the digester at the top thereof for delivering to said digester and exterior of the circulating system liquor from said system, a liquor supply pipe connected to the return pipe, valve means for establishing and cutting oil communication between the supply pipe and the return pipe, and valve means in the return pipe between the connection thereof to the supply pipe and the circulating system for establishing and cutting off communication between the latter and said return pipe.

ANDREAS CHRISTENSEN. 

